1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to methods for reducing the water content of such fibers.
2. Description of the Prior Art
As is known in the art, the water or OH.sup.- content of an optical waveguide fiber is one of the important parameters which determines a fiber's attenuation coefficient. Extensive efforts have gone into reducing fiber water levels so that today, fibers are expected to have water levels in the parts per billion range, whereas ten years ago, a level in the parts per million range was considered a significant achievement.
Although it is desirable to have a low water content throughout the fiber, it is especially desirable to have a low water content along the centerline of the fiber. This is particularly critical in the case of single mode fibers since for these types of fibers, the maximum field strength of the optical signal occurs at the centerline of the fiber and is concentrated in a small area.
Some of the techniques known in the art for reducing the water content of optical waveguide fibers are exemplified by U.S. Pat. No. 3,868,170 to Robert D. DeLuca and by U.S. Pat. No. 4,453,961 to George E. Berkey. The DeLuca patent describes depositing glass soot on a starting member to form a porous body, placing the body in a controlled environment having a water content of less than 20 parts per million, heating the body to a preselected temperature below the sintering temperature of the glass (e.g., 1,200.degree. C.), maintaining the body at the preselected temperature for a period of time sufficient for an equilibrium to be reached between the partial pressure of water in the body and the partial pressure of water in the controlled environment, sintering the body, and removing the starting member to form a consolidated tubular member which can be drawn into an optical waveguide fiber. Removal of the starting member can be performed either within or outside of the controlled environment. Similarly, the tubular member can be drawn into a fiber either within the controlled environment or in air.
The Berkey patent describes a method for preventing contamination of a glass blank, also referred to as a consolidated preform, having an aperture along its centerline. In accordance with this patent, the blank is formed by (1) applying particulate glass (soot) to the outside surface of an elongated mandrel; (2) removing the mandrel to form a porous preform having a longitudinal aperture down its centerline; and (3) drying and consolidating the porous preform following the procedures of U.S. Pat. No. 4,125,388 by surrounding the preform with a drying gas (e.g., 5% chlorine, 95% helium), passing drying gas through the preform's aperture, and heating the preform to a temperature above its sintering temperature. Once formed, the blank is sealed by closing one end of the aperture, evacuating the aperture, and then closing the other end of the aperture. In this way, the blank can be stored and subjected to further processing, including drawing, without fear of water or other contaminants entering the glass through the walls of the aperture.
In addition to the foregoing, U.S. Pat. No. 3,037,241 to Bazinet, Jr., et al, describes a rod-in-a-tube process for producing optical waveguide fibers wherein a vacuum is applied to the tube during drawing to remove air and gases from the space between the rod and the tube.
Although the process of the Berkey patent, supra, works successfully and can be used to produce optical waveguide fibers having a water content of less than 100 parts per billion (ppb), the blank sealing procedure of this patent adds a number of additional steps, and thus additional cost, to the overall waveguide production process. Also, fairly extensive training has been found necessary for workers to perform the sealing process successfully.
Accordingly, rather than sealing the blank after consolidation, an alternate procedure has been adopted wherein the blank is simply stored at room temperature with its ends open and a vacuum is applied to the aperture at the time of draw. It has been found that this procedure works successfully for blanks which can be stored at room temperature.
However, not all blanks can be stored at room temperature. Specifically, in order to achieve improved optical properties, blanks have been designed having chemical compositions which result in differences in expansion coefficients between different regions of the blank which are too great to allow the blank to be cooled to room temperature without fracturing.
For example, U.S. patent application Ser. No. 496,560, to V. A. Bhagavatula, which was filed on May 20, 1983 and which is entitled "Low Dispersion, Low-Loss Single-Mode Optical Waveguide," describes single mode optical waveguide fibers having refractive index profiles which result in improved dispersion and loss properties. Blanks suitable for making these fibers have thermal coefficients of expansion whose differences from region to region are on the order of 7.times.10.sup.-7 cm/cm.degree.C. Such blanks cannot be stored at room temperature without a substantial risk of fracture, e.g., approximately 75% of the blanks break, but rather must be stored at an elevated temperature.
When blanks of this type were stored with their ends open, as had been done with the blanks which could be stored at room temperature, it was surprisingly found that the water content of the finished fibers increased to unacceptable levels, e.g., to more than 100 ppb. This was found to be the case even though the same level of vacuum during drawing was used as had been used with the fibers which could be stored at room temperature. It was this problem of excessively high water levels for blanks requiring high temperature storage which led to the present invention.